Instructions
Collect a grenade. You start off with them in most multiplayer matches, but if not, you can find them lying around nearly everywhere. It doesn't matter if its a plasma or frag grenade.
Sneak up to the tank by clicking down on the left analog stick. Going into sneak mode will make you slower, but also make you invisible to radar.
Approach the Scorpion and hold "X" if playing "Halo 2" to board it. Hold "RB" to board it if playing "Halo 3" or "Halo: Reach."
Tap the "B" button to melee the vehicle. You can usually destroy the vehicle with three or four punches depending on how weak it is already.
Press the "L" trigger to plant a grenade. Your avatar will immediately jump off. Run away from the blast zone and watch the fireworks as the scorpion explodes.